You can also install the Unity launcher for it by running the following command:

sudo apt-get install go-mtpfs-unity

Before you try to mount the device, make sure it is unlocked (you'll know if it is or not if you see the open lock symbol at the bottom of the startup splash screen (underneath the Google logo) - if you don't see the symbol, it isn't unlocked).
Then connect your Nexus 7 via USB, and in the terminal type the following command: